Urine sample testing reliability in cannabis impaired driving cases

The police only took a urine sample from me. Is this enough to prove my THC blood concentration was above the legal limit?

A urine sample alone is generally not enough evidence to prove impairment and convict someone of a cannabis DUI. Since the police do not always take a blood sample from those they charge, many Ontario cases rely strongly on the results of urinalysis testing only. A lawyer can use this lack of evidence to defend the accused's charges in court.

The police and Crown Attorney will often try to rely on the evidence of a so-called Drug Recognition Expert (DRE) to prove impairment in cases where they do not have a blood sample. After making a demand for and obtaining a urine sample from the accused, it is tested for cannabis. The reason why this is unreliable is that cannabis can remain in a person’s body for upwards of 45 days. This is because it is a fat-soluble drug and not water-soluble like alcohol. Cannabinoids will be found in urine samples long after the intoxicating effects of the drug have worn off.

A urine test thus only really proves that a person consumed cannabis at some point in the last two months or so. No direct inference can be made about the degree of impairment, or the time of drug use based on these findings alone. Despite this, it is collected and analyzed in most cannabis impaired driving cases to show evidence of drug use. It is part of a package that the Crown will put together for the court to consider.

Since urinalysis alone does not provide conclusive proof, evidence of other signs of drug impairment are also relied upon by the Crown.

The DRE will conduct an interview and observational analysis of the accused upon arrest to note any signs of drug impairment. This test includes:

Eye exams: can the subject track and follow a stimulus, are their eyelids droopy, are there jerking motions, and inability to focus, dark room tests, pupil size, etc.

Dividend attention tests: including the Romberg balance test, heal-to-toe walk, standing on one leg

Interview, statements, and other observations: does the accused lose focus easily, slur, search for thoughts when talking

The drug impairment tests are virtually the same as the tests for alcohol impairment. These are largely subjective opinions of the police “expert” and the arresting officer on scene. Everyone reacts differently under stress particularly when facing potential criminal charges based on their behaviour. No case is the same.

Some people have medical conditions, are just tired, under the influence of prescription drugs, sick or dealing with an illness, and myriad other issues that can impact their performance on DRE tests for impairment. For those who do not speak English as a first language there may be communication problems or an inability to fully understand the questions and instructions. In short: DRE tests and signs of impairment are far from a perfect science. A perfectly sober person could fail such impairment tests.

How lawyers raise reasonable doubt as to the accused's THC blood level and degree of impairment.

Since a urine sample does not prove impairment or blood THC concentration level and DRE/police observations of impairment are subjective it is often not known for sure whether the accused actually drove while impaired. It is the job of the accused’s lawyer to present this reasonable doubt to the Crown Attorney and ask them to withdraw the charges (sometimes in exchange for an HTA offence like careless driving). If the Crown believes there is a reasonable prospect of conviction and the accused does not want to plead guilty, a judge will hear the evidence at a trial and decide.

Part of the job of the judge is also subjective as they must decide whether reasonable doubt exists or not. When there is no blood sample THC reading it works to the advantage of the accused because they can say they were not impaired and that the positive urinalysis was solely the result of prior use. Daily or habitual users of marijuana will always have a positive urine sample whether they are currently impaired or not.

Cannabis impaired DUI charges are often far more difficult to prove in court than people realize (especially if no blood samples were taken).

Some accused will wrongly equate a positive urinalysis with proof of impairment similar to a breathalyzer. They are completely different things. The police normally do not tell those who they charge with Criminal Code sections 320.14 (1)(c) and 320.14 (4) that the urine test is not definitive evidence of their blood THC level. Many accused think they have no chance of winning and plead guilty without realizing they actually have a very good case.

Since even the lowest possible sentence for cannabis impaired driving in Canada is a criminal conviction record an accused owes it to themselves to do everything they can to get the charges dropped or win an acquittal at trial.


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